Before 1970, there was no law in place regulating the amount of pollution that could be emitted by any single entity. However, in 1970, the Clean Air Act was passed and today there is clear evidence that it has indeed helped to reduce pollution.

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A collaborative project involving Jesse Nippert, a Kansas State University associate professor of biology, Richard Thomas, professor of biology at West Virginia University, Scott Spal, master's graduate from West Virginia University, and undergraduate Kenneth Smith from West Virginia University, shows that the Clean Air Act has helped forests systems recover from decades of sulfur pollution and acid rain. In four years, centuries of eastern red cedar tree, or Juniperus virginiana, rings were studied in the Central Appalachian Mountains of West Virginia. This region is of particular concern because it is directly downwind of the Ohio River Valley coal power plants and has experienced years of acidic pollution.

Red cedar trees were chosen because they are abundant, long-lived, and a good recorder of environmental variability. They grow slowly and rely on surface soil moisture, which made them sensitive to environmental change. According to Nippert, their abilities to live for centuries meant that researchers could analyze hundreds of years of tree rings.

According to the tree rings, scientists found that the trees have improved in growth and physiology in the decades since the Clean Air Act was passed. "There is a clear shift in the growth, reflecting the impact of key environmental legislation," Nippert said. "There are two levels of significance in this research. One is in terms of how we interpret data from tree rings and how we interpret the physiology of trees. The other level of significance is that environmental legislation can have a tremendous impact on an entire ecosystem."

The researchers analyzed the stable carbon isotopes within each tree ring as a recorder of physiological changes through time. Researchers analyzed tree rings back to the early 1900s, when sulfur dioxide deposition throughout the Ohio River Valley began to increase. By studying the stable isotopic signature in each tree ring, the researchers were able to compare the trees' growth patterns and changes in physiology to changes in atmospheric chemistry during the 20thcentury. Results showed that despite increased carbon dioxide -- which tends to increase plant growth -- tree growth and physiology declined for the majority of the 20th century when acidic pollution was high.

But scientists noticed a dramatic change around 1980, 10 years after the Clean Air Act was enacted.

"Our data clearly shows a break point in 1982, where the entire growth patterns of the trees in this forest started on a different trajectory," Nippert said. "It took 10 years for that landmark environmental legislation to reduce sulfur dioxide emissions, but it eventually did. When it did, we saw an entire ecosystem recover from years of acidic pollution."